Impermeable one-molecule thick barriers for ions and water have been produced on gold, using techniques of monlayer self-assembly (Sabatani et al., J. Phys. Chem. 91 6663-9 (1987)).
To confer ion-specificity to a metal electrode, the mere binding of ion-specific molecules to the electrode surface is not enough, as there must be prevented leakage of other, undesired species which might react electrochemically at the electrode.